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There are two methods used for SSB Transmission.
- Filter Method
- Phase Shift Method
This is the filter method of SSB suppression for the transmission. Fig
illustrates the block diagram of this method.
- A crystal controlled master oscillator produces a stable carrier frequency fc (say 100 KHz)
- This carrier frequency is then fed to the balanced modulator through a buffer amplifier which isolates these two satges.
- The audio signal from the modulating amplifier modulates the carrier in the balanced modulator. Audio frequency range is 300 to 2800 Hz. The carrier is also suppressed in this stage but allows only to pass the both side bands. (USB & LSB).
- A band pass filter (BPF) allows only a single band either USB or LSB to pass through it. It depends on our requirements.
Let we want to pass the USB then LSB will be suppressed. Inthis case.
fc = 100 KHz
Audio range = 300 - 2800 Hz
USB frequency range = fc + 300 to fc + 2800
= 100000 + 300 to 100000 + 2800
= 100300 to 102800 Hz
So this band of frequency will be passed on through the USB filter section
- This side band is then heterodyned in the balanced mixer stage with 12 MHz frequency produced by crystal oscillator or synthesizer depends upon the requirements of our transmission. So in mixer stage, the frequency of the crystal oscillator or synthersizer is added to SSB signal. The output frequency thus being raised to the value desired for transmission.
- Then this band is amplified in driver and power amplifier stages and then fed to the aerial for the transmission.
The advantages of single side band SSB transmission are as follows.
- It allows better management of the frequency spectrum. More transmission can fit into a given frequency range than would be possible with double side band DSB signals.
- All of the transmitted power is message power none is dissipate as carrier power.
- The noise content of a signal is an exponential function of the bandwidth: the noise will decrease by 3dB when the bandwidth is reduced by half. There fore, single side band SSB signals have less noise contamination than DSB double side band.
- The cost of a single side band SSB reciver is higher than the double side band DSB counterpart be a ratio of about 3:1.
- The average radio user wants only to flip a power switch and dial a station. Single side band SSB recievers require several precise frequency control settings to minimize distortion and may require continual readjustment during the use of the system.
Phase Shift Method
The phaseing method of SSB generation uses a phase shift technique that causes one of the side bands to be conceled out. A block diagram of a phasing type SSB generator is shown in fig.
- It uses two balanced modulators instead of one. The balanced modulators effectively eliminate the carrier. The carrier oscillator is applied directly to the upper balanced modulator along with the audio modulating signal. Then both the carrier and modulating signal are shifted in phase by 90o and applied to the second, lower, balanced modulator. The two balanced modulator output are then added together algebraically. The phase shifting action causes one side band to be canceled out when the two balanced modulator outputs are combined
- The carrier signal is VcSin2πfct the modulating signal is VmSin2πfmt. Balanced modualtor produces the product of these two signals.
Applying a trignometric identity.
(VmSin2πfmt)(VcSin2πfct) = 1/2[Cos(2πfc - 2πfm)t - Cos(2πfc + 2πfm)t]
Note that these are the sum and diffrence frequencies or the upper and lower side bands.
It is important to remember that a cosine wave is simply a sine wave shifted by 90o A cosine wave has exactly the same shape as a sine wave, but it occurs 90o
The 90o phase shifters create cosine waves of the carrier and modulating signal whichare multiplied in balanced modulator to produce
(Vm - Cos2πfmt)(VcCos2πfct)
(Vm - Cos2πfmt)(VcCos2πfct)
Another common trigonometric identity translates this to
(VmCos2πfmt)(VcCos2πfct) = 1/2[Cos(2πfc - 2πfm)t + Cos(2πfc + 2πfm)t]
Now if you add these two expressions together the sum frequencies cancel while the difference frequencies add producing only the lower side band:
Cos(2πfc - 2πfm)t